1. Field of the Invention
The invention relates to electron processing, and in particular to the sterilization of containers by energetic electrons.
2. Description of the Related Art
There are many applications of container sterilization (both for medical, pharmaceutical and food packaging) where the low temperatures involved with electron treatment are desirable. For example, for glass containers, superheated steam, flame or infra-red are used at temperatures from 275.degree.-400.degree. C. Such temperatures pose serious problems due to thermal shock. For example, using the Dole process, and non-pressurized superheated steam, the glass is elevated to 216.degree. C. and exits the sterilizer at an elevated temperature because of the high thermal capacity of the glass. It was found necessary to reduce the container temperature to a differential of 33.degree. C. between the container temperature at the point of fill and the cold sterile product. The speed and cost of heat removal makes such a process unacceptable for commercial use (see Von Bockelmann, B., Ch. 48, p. 841, "Aseptic Packaging"; "Disinfection, Sterilization and Preservation", ed: S. B. Block, Lea and Febiger, Phila Pa. (1991)).
For polymer containers made of blow molded plastics such as polyester or polypropylene, hydrogen peroxide in combination with heat or with high intensity ultraviolet irradiation, is usually employed.
The use of energetic electrons for the commercial sterilization of containers, such as those used for sterile product packaging in the pharmaceuticals and food industries, has been severely limited because of the inability of electrons of moderate energies (e.g. 100-300 keV) to penetrate the walls of the glass and blow-molded polymer containers typically used. The lower curve of FIG. 1 shows the end point of penetration or extrapolated ranges of electrons of varying energies in polyester (.rho.=1.40 g/cc). With unilateral irradiation, the electrons, once having penetrated the side walls, must be capable of reaching all regions of the container's interior, which may then involve air paths of 10-20 cm. The penetration in air at normal temperature (see Berger, M. J. and Seltzer, S. M., "Stopping Powers and Ranges of Electrons", NBSIR-82-2550 (1983), and pressure is shown in the upper curve of FIG. 1. Hence for typical polymer containers with wall thicknesses of 500-700 microns and practical diameters, electron energies well above 300 keV would be required for "through the wall" sterilization. Such energy sources are no longer easy to radiation shield for convenient, in-line use due to the penetrating bremsstrahlung generated at the higher energies (see Radiation Shielding, ed. J. K. Shultis and R. E. Faw, Prentice Hall PTR, Upper Saddle River, N.J. 07458 (1996)).
The term "electron opaque" is used in this teaching to describe container wall thicknesses which are at least 60% of the electron's extrapolated range in that material.
Based upon a knowledge of the depth:dose curves for energetic electrons, that is, the manner in which they deliver dose to matter as they near their range end points, one selects an energy for these applications for which the electron possesses an end point or range .about.1.6 times that required. For example, referring to FIG. 1, a typical 300 ml glass juice bottle has a depth of 14 cm. and a diameter of 6.5 cm. Hence a beam energy of 150 keV or more would be selected. For a 2000 ml polyester juice container with a depth of 25 cm and a diameter of 11 cm, a beam energy of 200 keV or more would be required.